Prototype biosensors have been designed for the detection of surface markers on bacterial cells and for carbon monoxide (CO). While these sensors were being engineered, sensitivity assays have been completed. These assays indicate that eight bacterial cells, as biosensors, can be detected in 30 ml of blood, and 50 cells detected in living tissue. These data indicate that these novel biosensors are exquisitely sensitive, perhaps allowing detection of several pathogenic bacteria in small blood samples. In a related project we have been able to non-invasively monitor host responses to systemic bacterial infections in the living rodent model for human typhoid fever. This further emphasizes the sensitivity of our approach.